Chemistry
Polarity
Polarity refers to the separation of electric charge within a molecule, leading to a positive and negative end. In chemistry, it is a fundamental concept that influences the physical and chemical properties of substances. Polar molecules have an uneven distribution of electrons, resulting in partial positive and negative charges, while nonpolar molecules have an even distribution of electrons.
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6 Key excerpts on "Polarity"
eBook - PDF- Des W. Connell(Author)
- 2005(Publication Date)
- CRC Press(Publisher)
Spatial analysis of the direction and strength of these bonds gives a resultant dipole moment of 1.80 D, which is in agreement 24 Basic Concepts of Environmental Chemistry with the experimental value of 1.87 D (Figure 2.5). As a result, water is a polar molecule with physical properties, and its ability to dissolve other substances is strongly influenced by its Polarity. 2.5 IONIC COMPOUNDS With some elements, the difference in electronegativities can be so great that one atom completely loses an electron to the other atom in the bond. For example, in the hydrogen chloride molecule, there can be a transfer of an electron from the hydrogen to the chlorine, resulting in the formation of a chloride ion with a full FIGURE 2.4 Chemical structures of some compounds with different polarities. Bonds and Molecules 25 negative charge and, consequently, a hydrogen ion with a full positive charge. This is illustrated below: In this transfer, the arrow indicates that both electrons that form the covalent bond have moved to the chlorine atom. In this situation, the chlorine atom in the HCl molecule, which held a small negative charge ( δ –), now becomes a fully negatively charged free atom, described as an ion. The covalent bond has been broken and no TABLE 2.4 Polarity of Common Groupings in Organic Molecules Highly Polar Polar Weakly Polar Nonpolar –COO – –O–H –CH 3 –O – –COOH . –CH 2 CH 3 –NH 3+ –NH 2 FIGURE 2.5 Representations of the water molecule and its polar characteristics. C Cl C Br C H C = O H Cl H Cl H Cl δ δ + + − − → → + – – 26 Basic Concepts of Environmental Chemistry longer exists (Figure 2.6). With the formation of ions, the hydrogen ion and chloride ion can move apart depending on conditions. In fact, in hydrogen chloride there is a mixture of polar hydrogen chloride molecules together with hydrogen and chloride ions. Measurements indicate that in pure liquid hydrogen chloride, 17% of the compound is in the ionic form.
eBook - PDFFoundations of Chemistry
An Introductory Course for Science Students
- Philippa B. Cranwell, Elizabeth M. Page(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
These features of a molecule are of fundamental importance and will be explained in the following section. 2.3.1 Electronegativity The electronegativity of an element is the power of an atom in a molecule to pull electrons towards itself when the electrons are in a bond. Linus Pauling devel-oped a scale that compares the relative electronegativities of elements with each other. Pauling ’ s scale has values from 0.7 to 4.0. The symbol for electronegativity Box 2.3 You may have noticed the phosphorus atom now has 10 outer electrons, which is two more than the accepted octet. Phosphorus is said to ‘ expand the octet ’ by accommodating more than eight electrons in its outer shell. The term for this behaviour is hypervalency, and several theories have been used to explain it. A discussion of hypervalency is beyond the scope of this textbook, but it should be noted that you could also encounter hypervalency when looking at bonding in the elements sulfur, chlorine, and iodine. 2.3 Polar bonds and polar molecules 57 is the Greek letter chi: χ . The more electronegative an element, the higher the value of χ . Fluorine is the most electronegative element with a value of 4.0; and francium, at the bottom of Group 1, is the least electronegative with a value of 0.7. Note that electronegativities are relative values, which means they don ’ t have units. Electronegativity varies across the periodic table, as seen in Figure 2.20. Moving across a period, electronegativity increases from Group 1 to Group 7 (17). From left to right across a period (row) of the periodic table, nuclei have an increasing number of protons; therefore, the force of attraction between the nucleus and outer electrons increases. This increased force of attraction means that the outer electrons are pulled in more strongly and the atomic radii decrease; so the nucleus exerts an increasing attractive force on any bonded elec-trons, and hence the electronegativity, or power of attracting electrons, increases.
eBook - PDF- Steven Murov(Author)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
They influence boiling points, solubility, chemical reactivity and are responsible for determining the 3-D structures of proteins and DNA. Bond Polarity can be predicted by an examination of Pauling's empirical values of electronegativities. One set of values for many elements are in the following chart: [Extracted from A. L. Allred, J. Inorg. Nucl. Chem. , 1961 , 17 , 215.] H = 2.20 Li = 0.98 Be = 1.57 B = 2.04 C = 2.55 N = 3.04 O = 3.44 F = 3.98 Na = 0.93 Mg = 1.31 Al = 1.61 Si = 1.90 P = 2.19 S = 2.58 Cl = 3.16 K = 0.82 Ca = 1.00 Ga = 1.81 Ge = 2.01 As = 2.18 Se = 2.55 Br = 2.96 Rb = 0.82 Sr = 0.95 In = 1.78 Sn = 1.96 Sb = 2.05 Te = 2.1 I = 2.66 Generally, when the electronegativity difference between two bonding partners is very small (such as for the carbon-hydrogen bond), the bond behaves as though it is non-polar. As the electronegativity difference increases, the bond Polarity increases to the limiting point where the bond is completely ionic. It is easier to just remember that bonds between identical elements and the important carbon - hydrogen bond are non-polar while most other nonmetal -nonmetal bonds are polar covalent. Metal to nonmetal and metal to polyatomic ion bonds usually have predominantly ionic character. For a molecule to be polar, there must be polar bonds and a lack of symmetry in structure such that the bond dipoles do not cancel each other out. CCl 4 has polar bonds but because of its tetrahedral geometry, is a non-polar molecule. Water has polar bonds and because of its bent geometry is highly polar. In general, polar molecules will have higher boiling points than non-polar molecules with similar molecular mass. Molecules with similar polarities will have a greater tendency to dissolve in each other or like dissolves like. Fig. 16-2 Copyright 2014 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part.
eBook - PDF- Dudley Williams(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
7. ELECTRIC PROPERTIES OF MOLECULES* A great many molecular phenomena have their origin in the electric properties of individual molecules. The most important electric proper-ties of a neutral molecule are its permanent dipole moment and polariza-bility. Through these the molecular motion is coupled to external electric fields giving rise to a host of effects including: absorption, emission, and scattering of radiation; refraction of light, polarization of dielectrics, and Stark effect. Absorption, emission and scattering of radiation were dis-cussed in Part 2. Refraction, polarization of dielectrics, and Stark effect are discussed in this part. Of the higher electric multipole moments only the molecular quadrupole can be singled out as producing observable effects. These are discussed in Chapter 7 .4. f An additional phenomenon which can be attributed to a characteristic of the molecular charge distribution which is not revealed by the lower multipole moments is optical activity. Optical activity is discussed in Chapter 7 .5. When atoms unite to form a molecule the atomic charge distributions are altered to such an extent that a permanent molecular dipole moment frequently results. This dipole moment is a vector quantity with com-ponents defined by equations of the form where qi is the charge on the ith particle, and Xi is its x coordinate. Mole-cules with permanent dipole moments are called polar molecules. If a polar molecule is placed in a static electric field it will be subject to a torque tending to align its dipole moment in the field direction. In addi-tion to exerting a torque on the molecule the electric field will slightly distort the molecular charge distribution and produce an induced dipole moment. The magnitude of this induced dipole moment depends on the structure of the molecule, the magnitude of the electric field, and the t See also Vol. 1, Chapter 8.5; Vol. 2, Section 10.6.3; and Vol. 6, B, Chapter 7.1.
eBook - ePub- Patrick E. McMahon, Rosemary McMahon, Bohdan Khomtchouk(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
EN) between the two bonded atoms. An approximate, arbitrary classification is:(∆EN) < 0.5 = weakly polar bond (∆EN) = 0.5 to 1.0 = moderately polar bond (∆EN) > 1.0 = strongly polar bondA convention used for analysis of complete molecules designates a polar covalent bond with a dipole arrow = (+ →); the “cross” end of the arrow is placed over the relatively positive (δ+) atom of the covalent bond, and the arrow point is placed over the relatively negative (δ–) atom of the covalent bond:Example: Determine the (∆EN) for the following bonds and classify the bonds as weakly polar, moderately polar, or strongly polar: C—H; C—O; O—H; H—F.Molecular Polarity measures the total vector sum of all bond polarities in a complete molecule; this is termed the molecular dipole moment. The dipole moment is proportional to the strength of each individual bond dipole and to the direction of each bond dipole distributed around a central atom. Bond dipoles pointing in the same direction add to produce a larger vector sum for the molecular dipole. Bond dipoles pointing in opposite directions partially or completely subtract (cancel out) to produce a smaller vector sum for the molecular dipole.The requirement for a molecule to be polar is that it must be electronically “lopsided,” referring to the distribution of electron density. Relating to mass density, a ball will wobble in flight only if the mass around the center is distributed asymmetrically; i.e., if the ball is lopsided. A ball, such as a baseball or golf ball, has several different material densities representing the core, the winding, and the cover. The balls are not lopsided because each of the materials is distributed symmetrically around the center. A central atom may show varying electron densities through polar bonds, but, if the polar bonds are distributed symmetrically around the central atom, the electron density will not be lopsided and the central atom will not contribute to Polarity.
eBook - PDF- Mircea S. Rogalski, Stuart B. Palmer(Authors)
- 2000(Publication Date)
- CRC Press(Publisher)
Chapter 8 D ielectrics 8.1. POLARIZATION OF DIELECTRICS 8.2. IONIC SOLIDS 8.3. FREQUENCY-DEPENDENT POLARIZABILITY 8.1. P o larizatio n of D ielectrics The dielectric properties of solids arise from the polarization produced by an electric field, and are described in terms of a specific function which must be added to both the field and the electric potential when the laws of electrostatics are applied to matter in the condensed state. As the solid is a large collection of ions and valence electrons, we differentiate between conductors , with a nearly-free flow of electric charge and dielectrics , having a negligible content of free electrons. In conductors the effect of applying an electric field is to redistribute the free charge until the internal field is cancelled, and hence, the net charge inside a conductor is zero under static conditions. When an electric field is applied to a dielectric, each nucleus of charge q = +Ze will tend to be displaced in the direction of the field, and the corresponding cloud of tightly bound electrons will more against the field, as illustrated in Figure 8.1 (a). The relative displacement, given by the position vector r of the centre of the negative cloud with respect to the nucleus, allows the restoring force due to the electrons to balance the force on the nucleus due to the applied field. As a result, an electric dipole moment p -q r is produced in each atom. As a consequence of the induced dipole moments, layers of negative and positive charges develop on the two surfaces of an element of dielectric, normal to the direction of the applied field E , as in Figure 8.1 (b). The elementary charge induced in a layer of thickness r = r and area dS can be expressed in terms of the polarized charge density o p =dql dS as follows: 311 312 Solid State Physics vv V J ( v which can be interpreted using P dS = P endS. + <^n ( a ) (b) Figure 8.1.
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